The present invention relates to vacuum brake booster for motor vehicles comprising a low pressure housing which is divided into a vacuum chamber and a working chamber by a movable rigid partition. The partition is connected to an axially movable control hub containing therein a valve arrangement with the control hub acting via a reaction device on a force transmitting element.
From French Pat. No. 2,118,925 a vacuum brake booster has become known in which the movable partition is fixed on the control hub. The control hub in the vacuum chamber has a shoulder around the circumference thereof and the partition positively abuts the shoulder in the direction of power delivery. By a circlip which positively engages the control hub, the partition and a rubber diaphragm adjoining the partition is held at the shoulder of the control hub opposite the direction of power delivery. The depth of engagement of the circlip into the control hub is such that it engages a groove of the valve piston of the valve arrangement and, thus, limits the axial motion of that valve piston.
The force transmitting element is coaxial of the control hub and sealingly guided out of the vacuum chamber and is supported by the front surface of the control hub by means of a rubber-like reaction disc. The reaction disc lies opposite to the valve piston with a small clearance and, thus, can receive the vacuum forces transmitted by the partition onto the control hub and also that part of the foot or pedal forces which are produced by the piston bar mechanically connected to the valve piston.
In an arrangement of this type the control hub must be constructed in such a way that it can transmit the vacuum forces produced onto the force transmitting element without being damaged. For this purpose, on one hand, an expensive resistant material having a high resistance to pressure must be used for the production of the control hub, and, on the other hand, the element of the control hub having the shoulder in the vacuum chamber must be made from such a strong material that the air channels arranged in the control hub do not create any critical weak areas regarding stress.
From German Pat. No. DE-AS 2,365,903 another vacuum brake booster is known in which the partition itself is constructed as the reaction device which adds up the vacuum forces and the foot forces and transmits them to the force transmitting element.
The force transmitting element is a stepped piston bar whose smaller diameter portion is guided in a bore of the valve piston of the valve arrangement.
The valve piston carries a circlip which comes into engagement with the end of the control hub facing the vacuum chamber and which axially secures the piston opposite the power direction. The partition is radially segmented, is supported by the valve piston and acts on the force transmitting element via a supporting plate which is slightly arched. The supporting plate abuts the step of the smaller portion of the force transmitting element in a form-locking manner and is kept in a defined position relative to the control hub and to the partition by spacers projecting through the partition. The supporting plate abuts the partition only by means of its outer circumference. By the rubber diaphragm which is buttoned onto a shoulder of the control hub and is fixed in the wall of the low pressure housing, the partition of the working chamber and vacuum chamber is effected. With operation of a pedal mechanically connected to the valve piston, the vacuum forces acting on the partition will act on the force transmitting element via the supporting plate. As a result the partition will conically deform and move the valve piston against the force of the pedal axially opposite to the power direction in order to transmit a reaction force relative to the output force onto the pedal.
In this brake booster the control hub is relieved to a large extent of the operating forces, since the partition is constructed as the reaction device. However, construction of the brake booster is complex due to the many components necessary.